Humoral memory, a key feature of adaptive immunity, is vital for vaccine protection against microbes but may also foster persistence of some auto-immune diseases. Long-lived plasma cells constitutively secrete epitope-specific antibody (Ab) after a first encounter with antigen (Ag), but after longer periods this population can need replenishment that is achieved by re-activation of Ag-specific memory B (Bmem) cells. T cell help and the germinal center (GC) reaction foster IgM+ and class-switched Bmem cells, and promote recall responses with high-affinity Ab. Among the potential fates of a B cell after its activation, remarkably little is known about how signaling within B cells regulates the formation, persistence, or reactivation of memory. In T cells, accumulating evidence suggests an interplay of cell metabolism with signaling and their functions and fates. The distinct biology of B cells an memory suggests that Bmem may use mechanisms different from T cells, yet very little is known about B lineage memory regulation. Target of Rapamycin (TOR), a major nexus for regulation of energy use in biosynthesis, is a serine-threonine kinase inhibited by the immune modulatory drug rapamycin. The goal of this research is to elucidate how mTOR regulates lymphocyte functions. TOR kinase participates in two functionally distinct adapter complexes, so that its enzymatic activity is acutely impaired in mTORC1 whereas the impact of allosteric inhibitors (e.g., rapamycin) on mTORC2 is time- and cell type-dependent. In an apparent paradox, rapamycin may block B cell proliferation and the generation of class-switched Ab, and yet re-vaccination responses in rapamycin-treated patients appear grossly intact. We have found that B lineage cells require expression of Rictor, a core component of mTOR complex 2 in B cells, to maintain the capacity for a recall Ab response. These and further findings lead us to hypothesize that Rictor, in part because it scaffolds mTORC2, promotes the generation or maintenance of Ag-specific memory B cells, and that this function is exerted at least in part through inhibition of FoxO1. To test the impact of mTOR on humoral memory, we have three specific Aims. The first is to elucidate the stages at which Rictor promotes protective Ab and humoral memory via T and B lineages (AIM 1). We also aim to determine B lineage-intrinsic molecular mechanisms whereby Rictor acts in B lineage recall (AIM 2). In parallel, we will use genetic approaches to determine what is the impact of mTORC1 on primary and recall Ab responses (AIM 3). We will identify mechanisms by which mTORC1 regulates the spectrum of Ig classes in immunity, and test a model that AID expression is enhanced by but not absolutely dependent on mTORC1, and is relatively rapamycin-resistant in Bmem cells. The expected outcome of the studies is that we will uncover novel but divergent roles for mTORC1 and 2 in determining the functional outcomes for B lineage cells and humoral immunity. In particular, we expect to find that mTOR acts to promote Bmem cells by mTORC1 & 2-dependent mechanisms, and that mTORC1 regulates not only B cell proliferation and clonal expansion, but the spectrum of switched Ig classes after immunization.